The present invention relates to an optical pickup device capable of detecting an aberration occurring in a light-concentrating optical system, an aberration correcting method and an aberration detecting unit for the pickup device.
An increase in recording density has been promoted by increasing the track recording density and narrowing the track pitch of the information recording layer of an optical disk. For this purpose, it is required to reduce the diameter of a light beam concentrated on the information recording layer of the optical disk. As a method for reducing the beam diameter, it has conventionally been promoted to increase the numerical aperture (NA) of a light beam emitted from the object lens of an optical pickup device for executing recording and reproduction on the optical disk and shortening the wavelength of the light beam.
However, when NA is increased to, for example, 0.85, it is difficult to construct the object lens of a single lens. As NA is increased, the fabrication tolerance and the assembly tolerance of the lens become severe. Therefore, the object lens constructed of a single lens is hard to make the aberration fall within the tolerance, and this disables beam diameter reduction. Accordingly, it has been attempted to combine the object lens with a hemispheric lens for the construction of an object lens with two lenses (coupled lens). This arrangement allows the fabrication tolerance and the assembly tolerance of the lens to be relieved and allows high NA to be achieved.
With regard to the optical disk, the information recording layer is covered with a cover glass in order to protect the information recording layer from dust and scratches. That is, a light beam emitted from the object lens passes through the cover glass and comes into a focus by being concentrated on the information recording layer located under the cover glass. When the light beam passes through the cover glass, a spherical aberration (SA) occurs. The spherical aberration SA is proportional to a thickness d of the cover glass and the fourth power of NA.SA∝d·NA4  (1)
The object lens is designed so as to cancel this spherical aberration, by which the spherical aberration of the light beam that has passed through the object lens and the cover glass is sufficiently reduced. However, if the thickness of the cover glass is deviated from a prescribed value, then a spherical aberration occurs in the light beam concentrated on the information recording layer, and the beam diameter is disadvantageously increased. Accordingly, there occurs the problem that the information cannot correctly be read and written. According to the expression (1), an error of the spherical aberration caused by a thickness error Δd of the cover glass is proportional to Δd and the fourth power of NA.
When the NA of the object lens is about 0.6 as in DVD (Digital Versatile Disc), the error of the spherical aberration caused by the thickness error of the cover glass is small, and the light beam concentrated on each information recording layer has been able to be concentrated on a sufficiently reduced spot.
Furthermore, a multi-layer optical disk formed by laminating information recording layers so that the increase in density of record information in the direction of thickness of the optical disk has already been commercialized as two-layer disk of DVD (Digital Versatile Disc). An optical pickup device for the recording and reproduction of the multi-layer optical disk as described above is required to concentrate the light beam into a sufficiently reduced spot on each information recording layer of the optical disk. However, the thickness from the surface of the optical disk to each information recording layer is varied every information recording layer. Therefore, the spherical aberration occurring when the light beam passes through the cover glass of the optical disk is varied every information recording layer. For example, a difference ΔSA between the spherical aberrations occurring in adjoining information recording layers is proportional to an interlayer distance t of the adjoining information recording layers and the fourth power of NA according to the expression (1).
In the case where the NA of the object lens is about 0.6 as in DVD, the difference ΔSA between the spherical aberrations of the information recording layers is small, and the light beam concentrated on each information recording layer has been able to be concentrated on a sufficiently reduced spot.
However, even if the thickness error Δd of the cover glass is same, a greater spherical aberration SA occurs as NA is increased. For example, an about fourfold spherical aberration SA occurs when NA=0.85 by comparison with the case where NA=0.6. Therefore, it is required to correct the spherical aberration caused by the thickness error of the cover glass according as NA is increased as in the case where NA=0.85.
Furthermore, even if the interlayer distance t between the adjoining information recording layers is same, a greater difference ΔSA of the spherical aberration occurs according as NA is increased. For example, a fourfold difference ΔSA of the spherical aberration occurs when NA=0.85 by comparison with the case where NA=0.6. Therefore, it is required to correct the difference ΔSA between the spherical aberrations of the information recording layers according as NA is increased as in the case where NA=0.85.
For example, Japanese Patent Laid-Open Publication No. HEI 11-195229 discloses an optical pickup device that corrects the aforementioned spherical aberration. This optical pickup device has a thickness sensor for measuring the thickness of the cover glass for the optical measurement of the thickness of the cover glass, corrects the spherical aberration based on the measurement result and sufficiently reduces the diameter of the light beam concentrated on each information recording layer of the optical disk.
However, the aforementioned optical pickup device has had the problem that it is required to be separately provided with the thickness sensor and the optical pickup device has been able to be neither compacted nor reduced in cost.
The object lens, which is constructed of two lenses, is required to be assembled so that the lens interval of the two lenses comes to have a prescribed value. However, if the lens interval is deviated from the prescribed value, i.e., if a lens interval error exists, then a spherical aberration occurs in the assembled object lens. Furthermore, if the two lenses have a lens thickness error due to the fabrication tolerance of the two lenses, then a spherical aberration occurs in the assembled object lens.
As described above, a spherical aberration occurs in the object lens due to the lens interval error and the lens thickness error. In general, the lens interval error and the lens thickness error are reduced to such an extent that no problem occurs when recording and reproducing information on the information recording layers of the optical disk. However, an expensive assembling apparatus is necessary for the assembling of the object lens so that the lens interval error is reduced, and a long time is required for the assembling. Furthermore, the manufacturing of the object lens of a small lens thickness error costs much.
As described above, there has conventionally been the problem that the optical pickup device can be neither reduced in cost nor manufactured in a short time.